You can look up the charts used by the IRS and see what amount will be withheld for what exemption amount. The chart uses whole dollar amounts and your payroll dept may use the percentage method but it will be very close -
Percentage method on pages 35-36 and tables starting on page 38. Take note of single vs married and weekly/semi-weekly/bi-monthly/monthly charts.
Using a 2010 form 1040 do a ballpark rough guesstimate of your taxable income for 2011, subtract the standard deduction ($5800 single/11,600 married) or your ballpark itemized deductions if larger, and $3700 for each exemption. Figure your tax and deduct any tax credits (Child/Education/Child Care/etc). For 2011 there will be no Making Work Pay credit or Residential Energy Credit.
Then divide your final guesstimate of your tax liability by the number of paychecks and you'll see what you need taken out for each paycheck.